1. Technical Field
The present invention relates to a positioning system which positions a current position based on signals from positioning satellites, a terminal apparatus, a control method of the terminal apparatus
2. Related Art
Conventionally, a GPS (Global Positioning System) system which positions a current position of a terminal apparatus using radio waves from satellites has been actually used.
In such a system, a terminal apparatus receives radio waves (hereinafter, referred to as satellite radio waves) from three or more of GPS satellites, and calculates distance (pseudo range) between each GPS satellite and the terminal apparatus. A current position can then be positioned based on the positions of each GPS satellite on satellite orbit and the aforementioned pseudo range.
To capture satellite radio waves, frequency of satellite radio waves to be received by the terminal apparatus must be expected in consideration of Doppler shift due to relative movement between each GPS satellite and the terminal apparatus, and the expected frequency must be searched.
However, frequency of a reference oscillator (also referred to as local oscillator) of the terminal apparatus may deviate from the reference frequency (hereinafter, referred to as frequency deviation of reference oscillator) due to change in temperature. Since a synchronizing purpose signal for synchronizing with satellite radio waves is generated based on frequency of the reference oscillator, frequency deviation of the reference oscillator causes frequency deviation of the synchronizing purpose signal. This causes deviation between an expected frequency of the satellite radio waves (which means frequency after a signal is received by an antenna of the terminal apparatus and then down-converted within the terminal apparatus) and frequency of the synchronizing purpose signal. Accordingly, if frequency deviation of the reference oscillator occurs, it takes longer time to capture satellite radio waves than the case where frequency deviation of the reference oscillator does not occur.
On the other hand, a technology to expect frequency error of the terminal apparatus using the frequency of transmitting signals from a cellular phone base station (hereinafter, referred to as base station), and use the expected frequency error to capture satellite radio waves has been proposed (e.g., JP-A-2002-228737 (FIG. 1 and the like)).
However, in the above-mentioned prior art, it is assumed that frequency of transmitting signals of a base station is managed with high accuracy. But the fact is that management of frequency accuracy of transmitting signals of the base station may be insufficient.
Accordingly, there is a problem that when frequency errors of the terminal apparatus are expected based on transmitting signals with original frequency errors of the base station, the expected values may include errors at the same degree as the frequency errors of the transmitting signals of the base station, and the time to capture satellite radio waves may thus not be sufficiently reduced.